Hot paint dip tank



March 29, 1960 H."H. JONES HOT PAINT DIP TANK Filed Oct. 2b. 195a VENTUR- ATTORNEYS.

HEAT EXCHANGE R United States Pa e HOT PAINT DIP TANK Harry H. Jones, Huntington Woods, Mich., assignor, by mesne assignments, to E. I. du Pont de Nemours and Company, a corporation of Delaware Application October 20, 1958, Serial No. 768,189 7 Claims. (Cl. 118-602) into a reservoir containing heating means for the paint.

A pump is provided to transfer the overflow paint from the reservoir back to the dip tank. The apparatus further provides cooling coils around the side walls of the dip tank above the paint level to condense solvent which is constantly evaporating from the surface of the hot paint.

The condensed solvent is recovered and charged back into the reservoir where it is mixed with the overflow and additional solids which must be added from time to time to make up those removed by the articles which are coated as they pass through the dip tank. The constant overflow features provides a uniform depth of a paint at all times. This is particularly important where articles to be dipped must be coated up to a prescribed height.

I have found that under high temperature operating conditions which provide the best film deposits on the articles being coated, some of the volatilized solvent will escape from the dip tank into the room. Trichlorethylene, the usual solvent used in hot dip paint, is very expensive and in higher concentrations may be toxic. Consequently, it is important to be able to recover substantially all of the solvent volatilized from the surface of the paint. It is also desirable to reduce the concentration of solvent vapors above the'paint level to prevent solvent wash of coated articles emerging from the paint. A heavy concentration of solvent vapors at the point where the articles emerge from the bath tends to redissolve the paintand reduce the thickness of the deposit. Obviously, it is desirable to avoid vapor Wash.

The primary object of the present invention is to provide an improved apparatus of this type wherein substantially all of the solvent volatilized may be recovered without the provision of any structure which interferes with the dipping operation.

Another object is to minmize the concentration of solvent in the space above the paint bath at the point Where coated articles emerge, thus reducing the vapor wash of painted articles.

A further object is to provide an improved overflow dam between the dip tank and the reservoir which minimizes turbulence and thus reduces the quantity of solvent vaporized.

Another object is to provide an apparatus wherein the temperature of the space above the paint level in the reservoir may be cooled to a temperature lower than that prevailing above the surface of the paint in the dip tank, thereby causing the solvent vapors to flow from the dip tank into the reservoir.

These and other objects and advantages of my invention will become apparent from the following description when read in conjunction with the accompanying drawings, wherein:

below =the. level' of the paint P.

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PaizentedMar. 29, 19

. Figure 1 is a semi-diagrammatic sectional view through the dipping apparatus of the invention;

Figure 2 is a view taken along the line 2-2 of Figure 1 showing means for raising and lowering the cooling coils in the reservoir; and

Figure 3 is a modified form of the invention of Figure 1 wherein the overflow from the dip tank is carried into the reservoir through a pipe.

In the preferred form of the invention cooling means is provided in the reservoir just above the paint level therein and below the paint level in the dip tank. This cooling means extends transversely of the apparatus and serves to condense very rapidly solvent vapors within the reservoir. As these solvent vapors condense, the space formerly occupied by the vapors is filled by additional vapors flowing over the bulkhead or dam between the dip tank and the reservoir. In other words,by eflicient condensation in the space above the paint level in the reservoir a draft is created, causing the vapors from the paint tank to flow' continuously into the reservoir where they are condensed and recovered. This substantially reduces the load on the condensing coils normally provided in the dip tank so that all of the vapors remaining are quickly condensed and their escape from the dip tank prevented. Furthermore, the concentration of vapors above the paint level inthe dip tank is reduced to reduce the tendency to vapor wash the painted articles emerging from the tank. a i

Referring to the drawing, the dip tank 10 is provided with a well 11 having sloping side walls 12, 14 at either end. An inclined plane 20 cooperates with the sloping side wall 14 to provide a dam over which paint flows from the dip tankvto the reservoir 18. A plate 21, vertically adjustable on studs 23, may be employed to facilitate lowering or raising the liquid level in well 11. In this formof the invention the reservoir is actually a part of the main tank 10 which is separated from the clipping well 11 by the dam 14, 20. The plate 21, which extends transversely of the apparatus, is lower than the level of the paint P so that the paint constantly overflows into the reservoir 18. The inclined plane 20 provides support for the overflow so that allsplashing and frothing is eliminated. By avoiding agitation of the overflow'which normally results when the paint flows over a vertical dam, much of the solvent which would otherwise be volatilized is retained in the paint. This is an important feature of my invention.' 7

Cooling coils 16 consisting of a pipe equipped with plate-like fins are mounted on the side wall of the tank 10 around the entire periphery thereof. A conduit 22 conducts paint from the reservoir 18 to heat exchanger 24 where the temperature of the paint is elevated to the desired point. A pump 26 serves to circulate the paint constantly from the reservoir to the heat exchanger and back into the well 11 through the pipe 28 which connects To provide the desired increased cooling in the reservoir, cooling coils 30 are mounted for vertical movement near the right end of the tank 10. There are four coils 32 extending through a series of fins 34 which ties them together and also serves to conduct heat from the surroundings to the cooled fluid which flows through the tubes 32. More or less coils may be used as required by the size of the tank and the temperature of the paint. The cooling coils 39 may be suspended from cables 4%, 42 which are entrained over the pulleys 44, 46, 48 and 50. Weights 52 and 54 connected to the opposite end of the cables 40 and 42 counterbalance the weight of the coils. By moving the weights up or down, the coils may be lowered or elevated. Long vertical screws or other suitable means may be used instead of the cable and pulley means. More intensive condensation is obtained when the coils 30 are at their lowermost point since the concentration of vapors is highest at the surface of the paint.

In order to recover solvent which is condensed onthe cooling coils 30 I have provided a trough 56 extending transversely of the apparatus directly beneath the coils. The trough 56 is attached to the end wall 13 of the tank 10. A drain pipe 58 conducts condensed solvent and any moisture that might be condensed along with the solvent to a separator tank 59 of conventional construction. Here the heavy solvent is continuously conveyed to the reservoir through a vented tube 63 while the lighter water overflows through the pipe 62. andis carried to the drain.

The cooling tubes 32 connect witha. source of cold fluid, preferably cold water,.througli. flexible conduits 36, 38 as shown in Figure 2. The flexible conduits permit the coils to move.

To further reduce vapor wash. it is desirable to include a pair of cooling coils 64.- mounted on. the longitudinal side walls of the dip tank directly above the apex .15 of the dam. It is at this end ofthe dip tank where the painted articles are removed from the paint bath. The additional cooling provided by the coils 64 insures removal of the painted articles to a solvent-free zone within a very short time after they have emerged from the bath.

It will be understood that effective condensation of the volatilized solvent is difficult because cooling means may be provided only around the inner periphery of the tank. Any cooling means which extends across the tank would interfere with the dipping of articles into the'paint bath. In accordance with the present invention, however, the rapid condensation efiected below the level of the paint in the dip tank causes the vapors to drain olf continuously into the reservoir and, consequently, accomplishes the same objective without placing cooling coils transversely across the dip tank.

In the modification of the invention shown in Figure 3, the sloping side wall 14 joins a vertical bulkhead 70 having a circular opening 72 in the upper portion thereof. The top edge 73 of the bulkhead 70 should extend only a short distance above the opening 72 so that it does not interfere with vapor flow into the reservoir. The opening 72 connects with a pipe 74 extending vertically down into the reservoir 18. The paint overflow is through the pipe 74. Since the paint is confined within the pipe as it flows into the reservoir, the opportunity for evaporation of thesolvent has been eliminated. This construction, therefore, further aids in' reducing the concentration of solvent vapors within the space above the paint level. Obviously, if the solvent is not evaporated in the first place it is not necessary to condense it.

Other modifications of this invention will be apparent to those skilled in the art. It is my intention to include within the scope of my invention those element's'which are equivalent or which may be reasonably construed to be embraced by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a dip painting apparatus comprising a dip tank adapted to be filled with hot paint containing a solvent which is volatile at paint temperature, cooling means above the paint level to condense vaporized solvent, a reservoir adjacent the end of said dip tank into which paint overflows from said dip tank, means for heating the paint and means for returning paint from the reservoir to the dip tank, the improvement comprising additional cooling means disposed within said reservoir above the level of the paint therein and below the level of the paint in the dip tank, whereby solvent vapors within said reservoir are rapidly condensed, thus causing solvent vapors above the paint in the dip tank to flow over the end of the dip tank into said reservoir.

2. The apparatus of claim 1 wherein said additional cooling means comprises water-cooled tubes mounted for vertical movement with respect to the paint level in said reservoir and extending across the apparatus.

3. The apparatus of claim 1 which includes a third cooling means immediately above the paint level in said dip tank near said end of the dip tank over which the paint flows.

4. A clip painting apparatus comprising a dip tank adapted to be filled with hot paint containing a solvent which is volatile at the paint temperature, a reservoir adjacent one end of said dip tank, an inclined plane at said one end extending from a point just below the normal paint level in said dip tank down into said reservoir, whereby overflow paint from said dip tank flows with a minimum of turbulence down said inclined plane into said reservoir and cooling means disposed in said reservoir above the paint level therein and below the paint level in the. dip tank to condense solvent vapors in said reservoir and cause solvent vapors above the paint in the dip tank to flow into said reservoir.

5. A dip painting apparatus comprising a dip tank adapted to be filled with hot paint containing a solvent which is volatile at the paint temperature, a reservoir adjacent one end of said clip tank, said dip tank being separated from said reservoir by a bulkhead containing an opening therethrough, a pipe connected to said opening on the reservoir side and extending downwardly into the reservoir, and cooling means disposed within the reservoir above the paint level therein and below the paint level inthe dip tank to condense solvent vapors in said reservoir and cause solvent vapors above the paint in the dip tank to flow into said reservoir.

6. The apparatus of claim 4 which includes a vertically adjustable plate mounted on said inclined plane and projecting above the top of the plane to facilitate adjusting the'paint level in the dip tank.

7. The apparatus of claim 5 wherein said opening is disposed just below the top edge of said bulkhead.

References Cited in the file of this patent UNITED STATES PATENTS 2,739,567 Jones et al May 27, 1956 

